Nature Bulletin No. 480-A February 10, 1973
Forest Preserve District of Cook County
George W, Dunne, President
Roland F. Eisenbeis, Supt. of Conservation
"Feathers make the bird" is an old saying that is scientifically accurate,
Any creature with a body covered by feathers must be a bird but let's
take a closer look. Feathers are unique. They represent one of the finest
works of design and engineering that nature has produced.
There is a lot of evidence that birds originated from reptiles. During the
Age of Reptiles there was a group of animals called "bird-footed
dinosaurs". They ran about on their hind legs which resembled the legs
of modern ostriches, and some of them were no larger than our
chickens. It is supposed that birds are descended from this group of
reptiles and that, as they became able to fly, their scales gradually
changed to feathers. It is not easy to see how a scale could become a
light flexible-web but the theory is that natural selection, after millions
of years, produced the miracle. Thenceforth, birds entered the realm of
the air to prey upon its teeming insects.
All we really know is that one day a strange fossil was found in a
limestone quarry at Solenhofen, Bavaria. There, among the remains of
dinosaurs that were buried by silt in a Jurassic sea, about 130 million
years ago, was a skeleton resembling that of a small dinosaur. But,
alongside it were the perfect impressions of wing and tail feathers. This
had been a bird ! The find was important because it proved what
scientists had always believed -- that birds came from reptilian
ancestors. They called it Archaeopteryx, which means "early wing", and
its feathers were as good as any that birds have today.
If you examine a feather closely through a strong lens or a microscope,
you will see that it is an amazing object. For instance, a flight feather of
a large bird -- like the wing-tip feathers of a turkey -- has a hollow
tapering shaft that is stiff but flexible. Branching from both sides of this
are "barbs". On both sides of each barb are branches fitted with hooks
and flanges with interlace so that the barbs are actually "zippered"
together. If you poke in one of its feathers, a bird can zip it closed again
with a deft touch of the bill. Another important feature is that the
feather's web is "cut on the bias", as a dressmaker would say. Like a
man's woven tie, it stretches in length or width when you pull on it but
springs back to the original shape.
The web on one side of the shaft of a flight feather is much narrower
than that on the other side. If this were not true, flight would be
impossible. In flying, birds do not reach forward with the wings and
pull backwards: they do not "row" through the air. The wing stroke is
up and down. That carried the bird forward because the broad web at
the rear of each flight feather meets more air resistance than the narrow
front web, and this causes the feather to rotate slightly in its socket as
the wing is raised and lowered -- something like the slats on a Venetian
blind -- downward on the upstroke and upward on the downstroke.
The large flight feathers and tail feathers are covered by much shorter
covert feathers that serve like shingles on a roof. The contour or body
feathers grow only on certain parts of the body and are scale-like in
shape. They shed rain and the air spaces between them insulate the bird.
When you see a sparrow sitting all fluffed out on a cold winter morning,
he is trapping layers of air inside his feathers to keep him warm.
According to Aristotle: "fine feathers make fine birds." Could be.
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Update: June 2012